Cisco ASR 9000 Series Software Manual page 41

Implementing Layer-3 Multicast Routing on Cisco IOS XR Software
MVPN over GRE
Native Multicast
GRE tunneling provides a method to transport native multicast traffic across a non-Multicast enabled IP
network. Once the multicast traffic in encapsulated with GRE, it appears as an IP packet to the core transport
network.
A GRE tunnel can be a forwarding interface when the router is the imposition (or encap) router for that GRE
tunnel. The imposition router must prepend a unicast IPv4 header and GRE header to the multicast packet.
The source and destination IPv4 addresses for the added header are determined by the user configuration of
the tunnel. The newly encapsulated packet is then forwarded as a unicast packet.
When a GRE tunnel is an accepting interface for a multicast route, the router is the disposition (or decap)
router for the tunnel. The outer IPv4 header and GRE header must be removed to expose the inner multicast
packet. The multicast packet will then be forwarded just as any other multicast packet that arrives on a
non-tunnel interface.
Forwarding behavior
Figure depicts a Unicast GRE tunnel between two routers. The imposition router has a multicast (S,G) route
which has the GRE tunnel as a forwarding interface. At the disposition router, the GRE tunnel is an accepting
interface for the multicast (S,G). As seen, the packet is unicast GRE encapsulated when it traverses the tunnel.
Figure 10: Unicast GRE tunnel between two routers
Starting with IOS XR 5.3.2 release, IPv6 traffic is supported.
Note
Cisco ASR 9000 Series Aggregation Services Router Multicast Configuration Guide, Release 5.1.x
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